The present invention relates generally to underground fluid conduits and more particularly to a composite shaft lining which is flexible and generally leak proof and which is formed of a concrete cylinder comprising individual concrete rings separated by joints with at least one supporting steel cylinder being connected thereto.
It has heretofore been known to utilize concrete cylinders and supporting steel cylinders connected together for lining shafts in water-bearing, loose-ground strata. Such linings generally act as composite linings for an underground shaft or formation.
As a rule, frictional forces will insure that a bond is established between the concrete cylinder and the outer steel cylinder which is arranged in the vicinity of the wall of the underground shaft. Forces from the ground or rock formation will press the steel cylinder and the concrete cylinder together so that there will not be created tangential or axial shifting movements between the steel and the concrete.
Furthermore, an inner steel cylinder may be arranged within the inner wall of the concrete cylinder. However, no frictional forces act between such an inner steel cylinder, which is usually arranged toward the center of the shaft and of the concrete cylinder, and this could lead to separation of the steel cylinder from the concrete cylinder. In this case, a bond may be usually insured by anchoring means which are welded to the outer surface of the steel cylinder and which engage the concrete cylinder.
The disadvantages of such shaft linings reside in the fact that the steel cylinder may be subjected to bending stresses as a result of the effects of working or excavating of the ground formation. Transmission of loads as a result of these forces from the steel cylinder to the concrete cylinder may frequently lead to cracks in the concrete cylinder as a result of tensile stress. Thus, the strength of the concrete cylinder will be significantly reduced and it will no longer be able to withstand the influences of ground or rock pressure and water pressure.
To avoid many of the disadvantages which arise, shaft linings have been developed which are constructed from an outer steel cylinder and an inner concrete cylinder, the outer steel cylinder being welded so as to be water-tight with the inner concrete cylinder being composed of individual rings or annular sections which are independent from each other. As a rule, the smooth steel cylinder is additionally provided with a bituminous coating on its inner surface in order to reduce the friction and thereby to allow relative movement between the concrete and the steel such as may occur during bending of the steel cylinder. However, such a measure significantly increases the amount of material used and it can be quite time consuming. On the other hand, it becomes necessary to utilize such measures in order to insure that no tensile stresses occur in the concrete cylinder since such stresses could lead to the formation of cracks or damage to the cylinder.
It has been found that in deep shafts which may run to a depth of up to 1,000 meters or more, this manner of construction is no longer useful because of the fact that there is encountered high water and rock pressure and resulting increase in the frictional forces developed. Since the steel cylinder which is rather thin does not contribute to the absorption of horizontal pressure, such pressure must be almost totally absorbed by the concrete rings. This may lead to a disproportionately large wall thickness of the concrete cylinder.
Accordingly, it is an object of the present invention to provide a shaft lining which will avoid many of the disadvantages of the known embodiments and which is suited for reduction of the relative movement between the steel cylinder and the concrete cylinder to as great a degree as possible. The invention seeks to attain these ends by a favorable design of the two cylinders and by insuring that the steel cylinder and the concrete cylinder are capable of withstanding loads from the rock or ground formation with the steel cylinder forming a water-tight seal against the wall of the underground shaft.